Zinc oxide (ZnO), like other transparent conducting oxides (TCOs) are important materials for-next generation optoelectronic devices such as low cost, optically transparent, and wavelength tunable LEDs, and large area flat-panel displays, and solar cells. Recently, significant efforts have been made directed to producing p-Type ZnO. M. Joseph et al., “p-Type Electrical Conduction in ZnO Thin films by Ga and N Codoping,” Jpn. J. Appl. Phys. 38, (1999) pp. L1205-1207; K. Minegishi et al., “Growth of p-Type Zinc oxide Films by Chemical Vapor Deposition,” Jpn. J. Appl. Phys. 36, (1997) pp. L1453-L1455; X. Gao et al., “Pulsed Laser Reactive Laser Deposition of p-Type ZnO Film Enhanced by an Electron Cyclotron Resonance Source.,” J. of Crystal Growth, 223 (2001) 135-139.
However, due to asymmetric doping limitations, ZnO remains difficult to dope p-Type. S. B. Zhang et al. J. Appl. Phys. 83, 3192 (1998). These difficulties result from low solubility of appropriate dopants, inability to form shallow acceptor levels, and dopant-induced self-compensation. Y. Sato et al., Thin Solid Films 281-282, 445 (1996).
Recently, reports have emerged indicating some success with N doping of ZnO from N2O or NH3 gas. Minegishi et al. reports using excess Zn in a CVD system to assist introduction N via NH3 into ZnO films. Following deposition, N doping is enabled by a high-temperature activation step which results in a reported hole concentration of 1.5×1016 cm−3 and mobility of −12 cm2V−1s−1. Joseph et al. reports using electron cyclotron resonance to assist in nitrogen doping via N2 or N2O, and also using co-doping techniques to increase the solubility of nitrogen in the ZnO film. This work report a relatively high hole concentration of 4×1019 cm−3 but at a very low mobility of 0.07 cm2V−1S−1.
Yan et al. in “Control of Doping by impurity Chemical Potentials: Predictions for p-Type ZnO,” Physical Review Letters 86, 5723 (2001) have proposed a model predicting that NO gas should be a better N dopant source than N2O and N2. This theoretical model indicates that the defect formation energy of N on an oxygen site (NO) from NO should be lower than that from N2O. Moreover, if deposition conditions that yield zinc-rich growth stoichiometries are used, the defect formation energy should be negative. Therefore, processes that produce these thermodynamically favorable conditions should allow N doping in ZnO from NO without the requirement of added energy (e.g., high-temperature or plasma) to break the NO bond. This process should also produce a higher solubility of nitrogen in ZnO, compared to using N2O, or N2.
U.S. Pat. No. 5,578,501 entitled “Method of Manufacturing a Solar Cell by Formation of a Zinc Oxide Transparent Conductive Layer,” Niwa; U.S. Pat. No. 5,420,043 entitled “Method of Manufacturing a Solar Cell,” Niwa; and U.S. Pat. No. 5,324,365 entitled “Solar Cell,” Niwa, each disclose methods for manufacturing solar cells in which a ZnO transparent conducting layer is used as electrodes. The ZnO layer used in these three patents are not made to be p-Type conductors.
A method for continuously depositing transparent oxide material (including ZnO) by chemical pyrolysis is disclosed in U.S. Pat. No. 5,180,686 entitled “Method for Continuously depositing a Transparent Oxide Material by Chemical Pyrolysis,” Banerjee, et al. This patent only disclose a method for making regular ZnO films for solar cell use, in which the ZnO is an n-Type conductor.
U.S. Pat. No. 5,612,229 entitled “Method of Producing Photovoltaic Device,” Yoshida, discloses a method for manufacturing solar cells in which a ZnO transparent conductor layer is used as electrodes. This patent does not disclose how to make p-Type ZnO and other metal oxide films.
U.S. Pat. No. 5,804,466 entitled “Process for Production of Zinc Oxide Thin Film, and Process for Production of Semiconductor Device Substrate and Process for Production of Photoelectric Conversion Device Using the Same Film,” Arao, et al., and U.S. Pat. No. 6,238,808 entitled “Substrate with Zinc Oxide Layer, Method for Producing Zinc Oxide Layer, Photovoltaic Device, and Method for Producing Photovoltaic Device,” Arao, et al., disclose methods of producing high quality ZnO films for use in solar cells. These patents do not disclose any method for making p-Type ZnO films.
Methods for manufacturing solar cells is disclosed in U.S. Pat. No. 5,716,480 entitled “Photovoltaic Device and Method of Manufacturing the Same,” Matsuyama, et al., and U.S. Pat. No. 5,913,986 entitled “Photovoltaic Element Having a Specific Doped Layer,” Matsuyama; however, these patents do not disclose methods for making p-Type ZnO films.
U.S. Pat. No. 5,458,753 entitled “Transparent Conductive Film Consisting of Zinc Oxide and Gallium,” Sata, et al., disclose better quality ZnO films containing Ga. The ZnO films are n-Type materials.
A method of producing n-Type ZnO film used as window layers in solar cells is disclosed in U.S. Pat. No. 6,040,521 entitled N-Type Window Layer for a Thin Film Solar Cell and Method of Making,” Kushiya, et al.
U.S. Pat. No. 5,990,416 entitled “Conductive Metal Oxide Film and Method of Making,” Windisch, Jr., et al., discloses a method to reduce a dopant in metal oxide films. The dopant is a metal element. This patent does not disclose any method for making p-Type ZnO films.
U.S. Pat. No. 5,078,803 entitled Solar Cells Incorporating Transparent Electrodes Comprising Hazy Zinc Oxide,” Pier, et al., discloses a method for manufacturing solar cells incorporating transparent electrodes comprising hazy ZnO. This patent does not disclose any method for making p-Type ZnO films.
A method for manufacturing a thin film photovoltaic device comprising a transparent conductive film, which may be ZnO is disclosed in U.S. Pat. No. 6,187,150 B1 entitled “Method for Manufacturing Thin Film Photovoltaic Device,” Yoshimi, et al. This patent does not disclose any method for making p-Type ZnO films.
U.S. Pat. No. 5,620,924 entitled “Method of Preventing Deterioration of Film Quality of Transparent Conductive Film,” Takizawa, et al., discloses a method of preventing deterioration of film quality of transparent conductive film, which may be ZnO. This patent does not disclose any method for making p-Type ZnO films.
A process for producing a thin film solar cell is disclosed in U.S. Pat. No. 6,242,687 B 1 entitled “Amorphous Silicon Photovoltaic Devices and Method of Making Same,” Schropp, et al. This patent does not disclose any method for making p-Type ZnO films.
U.S. Pat. No. 6,107,116 entitled “Method for Producing a Photovoltaic Element with zno layer having Increasing Fluorine Content in Layer Thickness Direction,” Kariya, et al., discloses a method for producing a photovoltaic element with ZnO as a layer having increasing F content in the layer thickness direction. This patent does not disclose a method for making p-Type ZnO films.
A method for manufacturing photovoltaic devices is disclosed in U.S. Pat. No. 6,043,427 entitled “Photovoltaic Device, Photoelectric Transducer and Method of Manufacturing Same,” Nishimoto. The method does not disclose making p-Type ZnO films.
A method for manufacturing photovoltaic devices comprising ZnO films is disclosed in U.S. Pat. No. 5,604,133 entitled “method of Making Photovoltaic Device,” Aoike. This patent does not disclose making p-Type ZnO films.
U.S. Pat. No. 4,612,411 entitled Thin Film Solar Cell with ZnO Window Layer,” Wieting, et al., discloses a method for producing thin film solar cells with ZnO window layers. This patent does not disclose methods for making p-Type ZnO films.
There is a need in the art of preparing transparent conducting oxides, and particularly ZnO, to reach the carrier concentration criteria for making LEDs.